Allow extracting the linear AEC output

This CL enables extracting the linear AEC output,
allowing for more straightforward
testing/development.

Bug: b/140823178
Change-Id: I14f7934008d87066b35500466cb6e6d96f811688
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/153672
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29789}
diff --git a/api/audio/echo_control.h b/api/audio/echo_control.h
index de80f50..b63f123 100644
--- a/api/audio/echo_control.h
+++ b/api/audio/echo_control.h
@@ -31,6 +31,12 @@
   // Processes the capture signal in order to remove the echo.
   virtual void ProcessCapture(AudioBuffer* capture, bool echo_path_change) = 0;
 
+  // As above, but also returns the linear filter output.
+  // TODO(peah): Make pure virtual.
+  virtual void ProcessCapture(AudioBuffer* capture,
+                              AudioBuffer* linear_output,
+                              bool level_change) {}
+
   struct Metrics {
     double echo_return_loss;
     double echo_return_loss_enhancement;